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剪切带是由高剪切应力引起的局部塑性区域。剪切带的形成能够适应材料的进一步变形，这可能会导致材料损伤的积累，以及流动应力的潜在减少。剪切带存在于晶体金属、陶瓷、金属玻璃和分子材料等多种材料类别中。总的来说，当材料受到局部剪切而发生软化时，通常会出现剪切带，形成更易受到材料快速流动影响的区域。这将导致局部的塑性不稳定性，降低材料的偏应力分量，并允许持续变形。分子固体通常被应用于冲击载荷等超高应变率环境中，由于其复杂的晶体堆积和晶体缺陷形成机制认识的不足，对剪切带形成的研究提出了巨大的挑战。分子晶体中初始的剪切带成核过程尚不明确。
Fig. 1 | Molecular center of mass renderings at 50 ps after maximum compression for all particle velocities. Only molecules with a shear strain above 0.15 are rendered. Molecules are colored by shear strain.
来自美国洛斯阿拉莫斯国家实验室理论部的Brenden W. Hamilton等人，对含能材料环三亚甲基三硝胺（RDX）中冲击波诱导的剪切带形成进行了分子动力学模拟，用于评估剪切带的成核过程。他们发现，在高压下，剪切带的初始形成位点（称为“胚胎”）在剪切带形成和生长前会大量形成并迅速降低偏应力，从而抑制了塑性变形。压缩状态释放后，这些胚胎愈合并恢复到材料的结晶相，形成一个可逆的过程。这与低压冲击明显不同：低压冲击中，塑性变形导致剪切带显著增长，且冲击释放后增长仍在继续。对剪切分子的聚类分析表明，剪切带是一个大型的、相互连接的网络。高压系统中会产生数百个小型、独立的团簇，其大小和剪切幅度并不会随着时间的推移而显著增长，但仍然会降低驱动塑性的剪切应力。总的来说，高压下剪切带的消失对RDX的机械强度和机械化学动力学至关重要。
Fig. 2 | Correlations between the shear strain at 2.5 ps after a material section is shocked vs. the shear strain of that material at 50 ps after maximum compression. Each point represents a 1 nm3 Lagrangian bin.
这些结果将对高爆炸药、高超声速系统等高应变率应用材料的模拟和开发产生广泛的影响。该文近期发表于npj Computational Materials 10: 147 (2024)，英文标题与摘要如下，点击左下角“阅读原文”可以自由获取论文PDF。
High pressure suppression of plasticity due to an overabundance of shear embryo formation
Brenden W. Hamilton & Timothy C. Germann
High pressure shear band formation is a critical phenomenon in energetic materials due to its influence on both mechanical strength and mechanochemical activation. While shear banding is known to occur in a variety of these materials, the governing dynamics of the mechanisms are not well defined for molecular crystals. We conduct molecular dynamics simulations of shock wave induced shear band formation in the energetic material 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) to assess shear band nucleation processes. We find, that at high pressures, the initial formation sites for shear bands, “embryos”, form in excess and rapidly lower deviatoric stresses prior to shear band formation and growth. This results in the suppression of plastic deformation. A local cluster analysis is used to quantify and contrast this mechanism with a more typical shear banding seen at lower pressures. These results demonstrate a mechanism that is reversible in nature and that supersedes shear band formation at increased pressures. We anticipate that these results will have a broad impact on the modeling and development of high-strain rate application materials such as those for high explosives and hypersonic systems.
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